A wide variety of radar systems are presently in use for locating target positions. For purposes of this application, the targets which are to be detected are capable of motion in three dimensions, for example, aircraft.
The location of an object capable of motion in three dimensions can be accomplished using a variety of techniques. By far the most popular technique in use today employs a two-dimensional radar, i.e., one locating a target in range and azimuth. Height information may be obtained by interrogating the target which generates a reply from the target including its height. Of course, this is possible only for those targets which are equipped with responding equipment.
Another approach which is successfully employed to locate targets in three-dimensions and which does not rely upon target carried equipment is a so-called 3-D radar.
Generically, 3-D radar designs must be a compromise. On the one hand, S/N ratios must be maximized to insure target detection. This effort generally is at odds with the desire to obtain high resolution. Accordingly, resolution, detection probability, or both, are less than what would be available in a 2-D radar system.
In some cases, two antennas are used, one sweeping in azimuth and the other simultaneously nodding in elevation. This arrangement has the obvious disadvantage of requiring two radar heads and the necessary associated equipment to produce the complex desired motion. Skolnik, in "Introduction to Radar Systems" (McGraw-Hill 1962), p. 459, suggests mounting a height finding antenna back-to-back with a search radar antenna. Milne, in "The Combination of Pulse Compression with Frequency Scanning for Three-Dimensional Radars" appearing in The Radio and Electronic Engineer, August 1964, pp. 89-106, suggests use of a linearly frequency modulated pulse to scan in elevation as the antenna scans in azimuth. While this would avoid the disadvantages of two radar heads it does not meet the other difficulty mentioned above.
The present invention overcomes these difficulties and achieves relative simplicity by employing a clip-on height finder antenna which is mounted on the back side of the 2-D radar reflector. Thus, the mounting of the clip-on height finder antenna provides the necessary azimuth sweep from the same pedestal and mounting components which sweep the 2-D or search antenna in azimuth. The invention also overcomes the necessity for mechanical components to nod the height finding antenna in elevation by employing a frequency scanned height finding antenna array which scans elevation as a function of transmitted frequency. Of course, the elimination of the mechanical components to nod the height finding antenna is not achieved without a corresponding "cost." The difficulty normally associated with frequency scanned radars is that the information to be extracted from the return signal is encoded in terms of frequency, and its time of receipt is further variable based upon the targets' range. This characteristic of the return signal, i.e., it is unknown in terms of frequency and time of receipt, is one of the main causes of complexity and corresponding expense in 3-D radars. Typically, plural receiver channels are employed, each covering a different frequency increment of the total required to scan the elevation area of interest. In this regard, see Long, U.S. Pat. No. 3,344,426. With this technique, while the number of receiver channels can be decreased, in an effort to reduce the cost and complexity, such reduction results in a direct reduction of accuracy and is therefore undesirable.
The simplification of pulse compression radar receivers has been a long standing goal. The prior art is typified by U.S. Pat. Nos. 3,774,201, 3,786,504 and 3,720,950, as well as "Putting the Squeeze on Radar Signals" by Collins in Electronics, January 1968, pp. 86-94. In this approach, the bandwidth of receiver components is reduced without reducing resolution by exchanging processing time for bandwidth, i.e., non-real time processing. The disadvantage of this approach is the requirement for additional signal processing. Whether this approach is successful depends on whether the trade-off between component bandwidth reduction is worth the added complexity occasioned by the need for the additional signal processing.
In accordance with the present invention, receiver complexity is reduced by taking advantage of a priori target range and azimuth information which allows reuse of a single receiver channel; indeed, under certain circumstances, only a single receiver channel is necessary. This is achieved by tuning the channel to the target's range prior to operation of the height finder. Were the target's range known precisely, only a single receiver channel would be necessary. However, inasmuch as the search radar and height finding radar are offset about 180.degree. in azimuth, their scans of similar search volumes are offset in time. The target's motion during this time period results in target range uncertainty and thus the necessity for multiple receivers. Nevertheless, the number of different receiver channels is vastly reduced over that required by prior art techniques.
A further advantage of the invention is its modularity. More particularly, the present invention allows the integration of a height finder of reduced complexity with a conventional 2-D search antenna, even after the conventional 2-D antenna has been in use for some time. This allows the user to upgrade his two-dimensional radar capability for full 3-D capability, but allows that decision to be postponed until such time as the additional capability is necessary, and further, does not penalize the user by requiring him to purchase either a stand-alone three-dimensional radar system or even a stand-alone height finding system.